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Chapter 1 of 12 • Page 1 of 248🔒 Protected PDF • Watermarked
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Circuit breaker
Isolator
Load break switch
None of the above
Isolator
An isolator is a mechanical switching device used for isolating a circuit or equipment from the live power supply for maintenance purposes. Unlike circuit breakers or load break switches, it is designed to be operated only when the current flowing through the circuit is zero or negligible (off-load condition).
An isolator is a mechanical switching device used for isolating a circuit or equipment from the live power supply for maintenance purposes. Unlike circuit breakers or load break switches, it is designed to be operated only when the current flowing through the circuit is zero or negligible (off-load condition).
Iswitching≈0 — The ideal operating current for an isolator.
The isolator provides a visible physical break in the circuit, ensuring safety during maintenance. Because it lacks arc-quenching chambers (unlike a circuit breaker), opening it under load would result in a severe, uncontrollable arc that could damage the equipment and jeopardize personnel safety. Therefore, the operating protocol mandates that the circuit breaker must be opened first to interrupt the current, followed by the operation of the isolator.
Isolators are essentially 'off-load' devices.
They are used for isolating a section of a transmission line or substation bus-bar for maintenance.
Isolators are often referred to as 'disconnecting switches'.
They ensure a visible break point for safety confirmation.
Provides a clear physical air gap
Simple and low-cost construction compared to circuit breakers
High reliability for isolation purposes
Cannot break fault currents
Cannot perform switching operations under load
Requires an interlocking mechanism with circuit breakers
Isolation of bus-bar sections
Isolation of circuit breakers and transformers for repair
Substation bus-switching operations
| Feature | Circuit Breaker | Isolator |
|---|---|---|
Interrupting Capacity | High (Fault current) | Negligible (Only charging current) |
Circuit breakers are designed for both normal load currents and heavy fault currents, requiring sophisticated arc-quenching media like SF6, vacuum, or oil.
The sequence of operation is always: Open Circuit Breaker -> Open Isolator -> Close Isolator -> Close Circuit Breaker.
B is correct — An isolator is an off-load switching device and therefore does not require a current rating for arc interruption, unlike circuit breakers.
Always remember the 'Isolator Interlock' rule: never operate an isolator until the associated circuit breaker has successfully interrupted the circuit flow.